Communicating to 4-wire RS-485

electro89

Member
Join Date
Mar 2012
Location
Canada
Posts
106
I'm trying to communicate an MVI71-MCM (Modbus Card for PLC5) as a master to some slave devices. The MCM Modbus ports can be set to 2-wire RS-485 or 4-wire RS-422. The slave devices are all 4-wire RS-485.
Is there any way to make these talk without the use of some kind of converter? Note: the distance is about 150ft.

This is how I connected the 2 RS-485 devices with no success:
Slave Master
4-wire RS-485 2-wire RS-485
TX+ --------------------\__ D+
RX+ --------------------/

TX- --------------------\__ D-
RX- --------------------/

I changed the wiring and changed the Master to read RS-422, and I started reading values. I know these registers are correct because I changed a value in the slave device and it changed in the PLC. Only problem is the numbers I am seeing in the PLC aren't nearly the same as they are on the slave. Should this configuration work?


Slave Master
4-wire RS-485 4-wire RS-422
TX+ -------------------- RX+
RX+ -------------------- TX+
TX- -------------------- RX-
RX- -------------------- TX+
 
Ignoring the problem with numbers for the moment, are you getting communication errors?

If not then you need to provide a partial list of expected values versus actual register contents. It may just be a information swap or interpretation problem.
 
I was not getting any errors, but I was finally able to solve the problem. I changed a setting in the slave device to send integer values, not floating point. Now I am seeing valid readings.

Thanks for the reply.
 

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